1. Field of the Invention
The present invention relates to hydrophilic precipitated silicas that are especially suitable for use in RTV-1K sealants, to a process for preparing them and to their use for thickening sealants.
2. Discussion of the Background
Sealants are elastic substances that are applied in liquid to highly viscous form for the sealing of buildings or installations against water, atmospheric influence or aggressive media.
Silicone rubbers are compositions which are convertible into the elastomeric state and comprise as their base polymers polydiorganosiloxanes containing groups amenable to crosslinking reactions. Suitable groups include, primarily H atoms, OH groups and vinyl groups, which may be located at the chain ends, or else may be incorporated in the chain. Incorporated into this system are fillers as reinforcers, their nature and amount significantly influencing the mechanical and chemical behavior of the vulcanizates. Silicone rubbers can be colored with inorganic pigments. One distinction is between high-temperature vulcanizing and room-temperature vulcanizing (HTV/RTV) silicone rubbers.
Among the room-temperature curing or vulcanizing silicone rubber compositions, it is possible to differentiate one-component (1K) and two-component (2K) systems. The first group (RTV-IK) polymerizes slowly at room temperature under the influence of atmospheric moisture, with crosslinking taking place through condensation of SiOH groups to form Si—O bonds. The SiOH groups are formed by hydrolysis of SiX groups of a species formed as an intermediate from a polymer with terminal OH groups and from what is called a crosslinker R—SiX3 (e.g. =—O—CO—CH3, —NHR). In two-component rubbers (RTV-2K) the crosslinkers used are, for example, mixtures of silicic esters (e.g. ethyl silicate) and organotin compounds, the crosslinking reaction that takes place being the formation of an Si O—Si bridge from ≡Si—OR and ≡Si—OH (—=methyl group; R=organic radical) by elimination of alcohol.
The thickeners used for RTV-1K silicone rubber include silicas. In view of the sensitivity to hydrolysis of the silicone sealants, these silicas must introduce as little moisture as possible into the system. For this reason, fumed silicas have been used almost exclusively to date for this application. Hydrophilic silicas have not been used to date, on account of their high moisture content.
U.S. Pat. No. 5,395,605 claims precipitated silicas with a water absorption (23° C. and 64% relative humidity) of less than 4% whose tapped densities are above 100 g/l. The silicas from U.S. Pat. No. 5,395,605, however, can be used only in HTV silicone rubber and in cable extrusion. They are unsuitable for use in RTV1 silicone rubber, since the tapped density is too high and hence the thickening (yield point) would come out at too low a level. The process of U.S. Pat. No. 5,395,605 has the disadvantage that the silicas have to undergo a very long thermal treatment at very high temperature. As a result, the process is of little economic interest. In addition, the temperature treatment of U.S. Pat. No. 5,395,605 causes hardening and baking of the silicas, thereby making it mandatory for the silicas to be ground after thermal treatment.
WO 2005061384 shows the preparation and use—including use in silicone rubber—of silicas which according to the claim have a water absorption of <6% and a DOP>300 ml/100 g. The silicas disclosed in the examples of WO 2005061384, however, all have a water absorption of between 5.7% and 5.9% and are therefore unsuitable for use in RTV-1K formulations. Accordingly, WO 2005061384 describes only their use in silicone rubber formulations for extrusion processes (HTV).
EP 1557446 describes exclusively HTV silicone rubber formulations. The silicas employed therein have a loss on drying of <4%. Data on the equilibrium moisture content of the silicas used after exposure to high atmospheric humidity (this should not be confused with the loss on drying) are as absent from EP 1557446 as indications that the silicas described therein can be used in RTV silicone rubber formulations. The formulations disclosed in EP 1557446 are used for producing insulating materials such as cable sheathing, for example.
In summary, therefore, it can be stated that the above patents do not disclose any hydrophilic precipitated silicas which meet the exacting requirements for use in RTV-1K silicone rubber. There is therefore a strong need for hydrophilic precipitated silicas of this kind that are suitable for RTV-1K applications.